Fix potential freeing of an immutable static buffer

strerror_r() has two versions: GNU-specific and XSI-compliant. The XSI
version always stores the string in the provided buffer, but the GNU
version might store it in the provided buffer or it might use some
immutable static buffer instead. Since we always free the error string,
we might end up freeing the immutable static buffer.

other/bootstrap_daemon/docker/tox-bootstrapd.sha256

@@ -1 +1 @@
-de00572e0a22b67defb05759a4d5aac6bf0e107bfd6834a1edc20ffb0379528d  /usr/local/bin/tox-bootstrapd
+746158481ebd16d70aadc0bf4d2dc6da6a2f3ac4eb12d219b49fc6fd7e60d149  /usr/local/bin/tox-bootstrapd

toxcore/network.c

@@ -1746,18 +1746,36 @@ char *net_new_strerror(int error)
                    error, 0, (char *)&str, 0, nullptr);
     return str;
 #else
-    char *str = (char *)malloc(256);
+    char tmp[256];
+
+    errno = 0;
+
 #ifdef _GNU_SOURCE
-    str = strerror_r(error, str, 256);
+    const char *retstr = strerror_r(error, tmp, sizeof(tmp));
+
+    if (errno != 0) {
+        snprintf(tmp, sizeof(tmp), "error %d (strerror_r failed with errno %d)", error, errno);
+    }
+
 #else
-    const int fmt_error = strerror_r(error, str, 256);
+    const int fmt_error = strerror_r(error, tmp, sizeof(tmp));
 
     if (fmt_error != 0) {
-        snprintf(str, 256, "error %d (strerror failed with error %d)", error, fmt_error);
+        snprintf(tmp, sizeof(tmp), "error %d (strerror_r failed with error %d, errno %d)", error, fmt_error, errno);
     }
 
+    const char *retstr = tmp;
 #endif
 
+    const size_t retstr_len = strlen(retstr);
+    char *str = (char *)malloc(retstr_len + 1);
+
+    if (str == nullptr) {
+        return nullptr;
+    }
+
+    memcpy(str, retstr, retstr_len + 1);
+
     return str;
 #endif
 }